Apparatus for measuring wavefront aberrations, especially wavefront aberrations of lenses or the surface precision of spherical surfaces using interference fringes have been known for some time. Such apparatus are indispensable in the examination of high-precision lenses.
Many different types of schemes have been proposed for improving the measuring precision of such apparatus when measuring wavefront aberrations from the geometric strength of the interference fringes. Examples of such proposals are disclosed in Japanese laid-open patent documents Sho 62-127601 and Hei 7-16730.
With the aforementioned conventional technologies, it is assumed that aberrations in the optical system within the interferometer (specifically, interference-fringe distortions arising from any aberrations of a converging lens or lens system located between the measured object and an image pick-up element) is negligible. Such an assumption leads to problems of a loss of measuring precision when measuring spherical surfaces.
For example, the Japanese laid-open patent document Sho 62-127601 cited above discloses methods for removing measurement errors whenever the curvature enters of the reference surface and the measured surface become defocused (due to shifts in the direction of the optical axis); however, the converging lens in the interferometer is assumed to satisfy the sine condition. I.e., it is assumed that the optical system does not impart any interference-fringe distortion. Hence, with the conventional technologies, whether or not interference-fringe distortion can be corrected has had a significant effect on measurement precision.
With the conventional technology summarized above, there are problems in providing a converging lens that satisfies the sine condition. This problem is especially acute when using a Fizeau-type interferometer where, because the last lens surface of the Fizeau assembly is used as a reference surface ("Fizeau surface"), there are more optical design restrictions and the amount of interference-fringe distortion arising from converging-lens aberration increases. Such distortion is extremely difficult to correct.